Scanners with micromirrors (MEMS scanner) frequently consist of a mirror plate, suspended by springs so as to be movable in one or more axes, which plate is driven by an electrostatic, electromagnetic, thermal, or piezoelectric transfer of force. Laser scanners of this type are used in the field of measurement engineering, for example, in microscopy, in optical coherence tomography, in light barriers, in distance measurement, in profilometers, in fingerprint sensors, etc., for example, but also in consumer applications such as laser video projectors, mobile telephones, laptops and MP3 players.
One very advantageous type of laser projection is based upon resonant operation of the MEMS scanner, because in this case a favorable amplification of the mirror oscillation amplitude can be exploited with simultaneously low power consumption. This applies to both single-axis and multiple-axis MEMS scanners.
One particularly advantageous configuration of such a resonant MEMS scanner provides for operation of the actuator at diminished pressure (vacuum), because this allows damping to be very substantially reduced. This can be exploited to allow the scanner to be operated at minimal power consumption, which is highly significant for all mobile applications (mobile telephone, MP3 players, etc.). With low damping, significantly higher resonance amplitudes of the scanner can be achieved than with conventional resonators operated at atmospheric pressure. Advantages of MEMS laser scanners operated in a vacuum over non-packaged scanners are: greater achievable scanning angles, lower power consumption by several orders of magnitude, higher usable scanning frequencies, lower drive voltages (with electrostatic or piezoelectric actuators).
To generate a vacuum of this type for each of the scanners, which are customarily produced as a plurality on silicon wafers, the scanners can be encapsulated with a hermetic seal inside the wafer laminate, in other words, before being separated into chips, by bonding one glass wafer to the front and bonding a second wafer to the back against the MEMS wafer. A getter encapsulated within the cavities produced in this manner, e.g., a titanium layer, which is deposited on the rear wafer, can chemically bind the majority of encapsulated gas molecules by activation at the appropriate temperature, thereby enabling a minimal gas pressure, which is limited essentially only by the diffusion of natural helium contained in the ambient air back through the glass cover, which is permeable to helium. In the end, this results in an establishment in the cavity of the natural helium partial pressure.
As was already mentioned above, vacuum packaged resonant MEMS laser scanners of this type are of particular interest for mobile laser projection displays in mobile telephones, in which the main goal (for the purpose of the longest possible projection playback time, as is necessary, for example, for playing back movies) is to require that the lowest possible amount of power consumption be reserved for the scanner. Although most implementations of such MEMS-based laser projection displays have heretofore involved a raster-type scanning method, in which one biaxial scanner or two uniaxial scanners arranged in a row are equipped with both a resonantly and a non-resonantly operated scanning axis, advantageous power-saving operation using a vacuum packaged MEMS scanner can be achieved only with the resonant operation of both scanning axes. As a consequence, this results in a Lissajous-type scanning method, in which the pixels generally are not read out and projected from the image memory in the sequence in which they arrive in the image memory.
Because the goal with laser projection displays, particularly for applications in the field of consumer products, is to devise extremely economical systems, the smallest possible component must be devised, while maintaining all other optical/electrical/mechanical requirements. The smaller the component (of the MEMS scanner chip), the more components will fit on one silicon wafer, and therefore, the more economically the component can be manufactured. For this reason, it is expedient to accommodate both scanning axes on only one chip. Conventional 2-D scanner chips are equipped with a mirror plate for this purpose, which is movably suspended in a second frame, which is also movably suspended. This arrangement is frequently identified by the technical term “gimbal mounted scanner”, and corresponds approximately to the term “gimbal suspension”. The two scanning axes are oriented orthogonally to one another. The gimbal assembly allows the set movements of both axes to be generally effectively isolated from one another. This means that each axis can be operated separately, largely without influence by the operation of the respectively other axis. Because a laser projection display generally processes the image data in a sequential line orientation, there ordinarily is a fast axis, which effects the horizontal deflection (“line sweep”), and a scanning axis, which generates the vertical deflection at a lower frequency. Therefore, the inner suspension of the mirror is ordinarily designed such that the resonance frequency lies between 16 kHz and 32 kHz, whereas the frame that surrounds the mirror forms the slow scanning axis, and has a substantially lower resonance frequency (“Wafer-level vacuum packed micro-scanning mirror for compact laser projection displays”, PROC. SPIE, 2008, Vol. 6887, 688706-1 to -15).
However, there are critical disadvantages to this gimbal-suspended, biaxial, resonantly operated gimbal scanner.
1. As a result of the movable frame which surrounds the mirror plate, the space required by the scanner is necessarily much greater than for the mirror plate with its suspension mounts alone. This circumstance is especially problematic in the case of scanners for very high resolution, because high resolution for the scanner means that a very large scanning angle is required, which can in turn be realized only by means of long suspension mounts, due to the mechanical stress in the suspension. The surrounding gimbal must also be correspondingly large in design. Therefore, the gimbal scanner may be a design that is too costly for the application.
2. The requirement of high resolution results in both high oscillation amplitudes and always a very high scanning frequency requirement for the fast axis. Typically, frequencies greater than 30 kHz are necessary for high-resolution Lissajous projection. The large scanning angles with such high scanning frequencies are equivalent to high occurring rotational accelerations, and, as a result of the maintenance of angular momentum, frequently result in a substantial mechanical overcoupling to the surrounding movable frame: The rapid rotational movement of the inner mirror axis transmits so much restoring moment to the gimbal structure that said structure resonates vertically in the opposite direction, i.e., in phase opposition. As a result of this resonance, the movable drive electrodes of the gimbal are also raised out of their set position and, when acted upon by the drive potential, generate a different electrostatic torque than was actually planned. Frequently, this mechanical/electrical overcoupling mechanism results in unstable behavior of the actuator and a deviation of the movement from the set function, which can be detected only imprecisely by the position sensing system, with the result that image interference becomes visible. The amplitude of this undesirable overcoupling movement of the gimbal is greater, the lower the moment of inertia of the gimbal is. Therefore, in order to keep the amplitude negligibly low, a gimbal of sufficiently large diameter, large mass and/or high moment of inertia is required. Ultimately, this again means an unfavorably large and therefore cost-intensive chip.
3. The large gimbal mass that is required for minimizing mechanical overcoupling results in a high sensitivity to external vibrations and shock effects, and therefore, to reduced MEMS sturdiness. A MEMS scanner with a gimbal suspension therefore always represents a compromise between sturdiness and a minimization of overcoupling.
4. In order to generate high levels of torque, the comb electrodes of the electrostatic drive must be attached at a maximum distance from the rotational axis. However, in the case of a gimbal scanner, this causes stator electrode and rotor electrode to leave the area of electrode overlap, even with relatively small scanning angles, and then to transmit only a small amount of power. In principle, an arrangement in which the interelectrode distances are small during the intermittently necessary generation of power and/or torque would be more advantageous.
5. In terms of design, a gimbal MEMS scanner has a plurality of parasitic eigenmodes, which influence the desired set movement more significantly the greater the scanning angles, i.e., actuator deflections, to be achieved, because large scanning angles necessitate long suspension mounts, and long suspension mounts produce more disruptive eigenmodes in the vicinity of the useful oscillation than short suspension mounts. Due to the concertinaed arrangement of two resonators and due to the necessarily large dimensions, the gimbal suspension exhibits an abundance of parasitic modes, in which parts of the outer resonator and parts of the inner resonator are involved in combination. The MEMS designer is therefore always required, despite the less favorable characteristic frequency spectrum, to find a suitable tuning which supplies two useful eigenmodes having the least possible interference for the scanning axes.
6. The consequence of the alternating thermal loads induced by the laser for scanning, particularly in the case of video laser projection, is that the resonance frequency of the MEMS mirror can be shifted within a very short time, resulting in phase and amplitude modifications, and ultimately leading to image defects. In the case of a gimbal scanner, the thermal insulation effect is relatively high, as a result of which heat intake accumulates unfavorably on the thin torsion springs. The reason for this is that two suspension pairs, those of mirror and gimbal, are connected in series before the heat can be released via thermal conduction to the solid chip frame. The problem is generally further intensified by the fact that a fast axis and a slow axis are combined with one another. The slow axis is ordinarily implemented as a very thin, and therefore non-rigid, spring suspension, which impedes the removal of heat. Shorter distances having a larger overall cross-section between mirror plate and chip frame would be more advantageous for avoiding amplitude and phase fluctuations.
The problem addressed by the invention is therefore that of providing a deflection device for a scanner having Lissajous scanning and resonant operation, which minimizes or avoids the above-mentioned problems, and which, despite Lissajous scanning, provides effective image overlap and scanning resolution.
This problem is solved according to the invention by the characterizing features of the main claim in combination with the features of the preamble.
With the measures specified in the dependent claims, advantageous further developments and improvements are possible.